Social interaction is usually discussed in terms of the exchange of communicative gestures and other special signals. In a long-established captive group of monkeys, however, or in the wild, animals very often change their position in response to the movement of others without any apparent exchanges of overt communication. We think, in fact, that much signal communication, especially aggression and submission, only occur when basic distance-regulating behaviour break down. Thus, understanding this latter will provide new insights into the mechanisms of social organization in stable groups of primates. We have identified three types of such "spatial interactions" which we call anticipators-avoid, monitor-adjust, and synchrony. These will be described for patas monkeys and their occurrence related to group density in the caged and in a free-ranging group, and to the characteristics of the individuals involved. The effect of recent experience and observations of other monkeys will also be explored. Comparisons will be made with groups of other monkey species housed in comparable conditions to verify possible species typical synchrony levels.